Conventional electrochemical fuel cells convert fuel and oxidant into electrical energy and a reaction product. A typical layout of a conventional fuel cell 10 is shown in FIG. 1 which, for clarity, illustrates the various layers in exploded form. A solid polymer ion transfer membrane 11 is sandwiched between an anode 12 and a cathode 13. Typically, the anode 12 and the cathode 13 are both formed from an electrically conductive, porous material such as porous carbon, to which small particles of platinum and/or other precious metal catalyst are bonded. The anode 12 and cathode 13 are often bonded directly to the respective adjacent surfaces of the membrane 11. This combination is commonly referred to as the membrane-electrode assembly, or MEA.
Sandwiching the polymer membrane and porous electrode layers is an anode fluid flow field plate 14 and a cathode fluid flow field plate 15. Intermediate backing layers 12a and 13a may also be employed between the anode fluid flow field plate 14 and the anode 12 and similarly between the cathode fluid flow field plate 15 and the cathode 13. The backing layers are of a porous nature and fabricated so as to ensure effective diffusion of gas to and from the anode and cathode surfaces as well as assisting in the management of water vapour and liquid water. Throughout the present specification, references to the electrodes (anode and/or cathode) are intended to include electrodes with or without such a backing layer.
The fluid flow field plates 14, 15 are formed from an electrically conductive, non-porous material by which electrical contact can be made to the respective anode electrode 12 or cathode electrode 13. At the same time, the fluid flow field plates must facilitate the delivery and/or exhaust of fluid fuel, oxidant and/or reaction product to or from the porous electrodes. This is conventionally effected by forming fluid flow passages in a surface of the fluid flow field plates, such as grooves or channels 16 in the surface presented to the porous electrodes 12, 13.
With reference also to FIG. 2(a), one conventional configuration of fluid flow channel provides a serpentine structure 20 in a face of the anode 14 (or cathode 15) having an inlet manifold 21 and an outlet manifold 22 as shown in FIG. 2(a). According to conventional design, it will be understood that the serpentine structure 20 comprises a channel 16 in the surface of the plate 14 (or 15), while the manifolds 21 and 22 each comprise an aperture through the plate so that fluid for delivery to, or exhaust from, the channel 20 can be communicated throughout the depth of a stack of plates in a direction orthogonal to the plate as particularly indicated by the arrow in the cross-section on A-A shown in the FIG. 2(b).
Other manifold apertures 23, 25 may be provided for fuel, oxidant, other fluids or exhaust communication to other channels in the plates, not shown.
The channels 16 in the fluid flow field plates 14, 15 may be open ended at both ends, ie. the channels extending between an inlet manifold 21 and an outlet manifold 22 as shown, allowing a continuous throughput of fluid, typically used for a combined oxidant supply and reactant exhaust. Alternatively, the channels 16 may be closed at one end, ie. each channel has communication with only an input manifold 21 to supply fluid, relying entirely on 100% transfer of gaseous material into and out of the porous electrodes of the MEA. The closed channel may typically be used to deliver hydrogen fuel to the MEA 11-13 in a comb type structure.
With reference to FIG. 3, a cross-sectional view of art of a stack of plates forming a conventional fuel cell assembly 30 is shown. In this arrangement, adjacent anode and cathode fluid flow field plates are combined in conventional manner to form a single bipolar plate 31 having anode channels 32 on one face and cathode channels 33 on the opposite face, each adjacent to a respective membrane-electrode assembly (MEA) 34. The inlet manifold apertures 21 and outlet manifold apertures 22 are all overlaid to provide the inlet and outlet manifolds to the entire stack. The various elements of the stack are shown slightly separated for clarity, although it will be understood that they will be compressed together using sealing gaskets if required.
In order to obtain high and sustained power delivery capability from a fuel cell, it is generally necessary to maintain a high water content within the membrane-electrode assembly, and in particular within the membrane.
In the prior art, this is conventionally achieved by humidifying the feed gases, either fuel, air or both, fed via manifolds 21, 22 or 23 and channels 16. A disadvantage with this technique is that in order to maintain sufficient humidification levels, the inlet gas streams often require heating and supplementary apparatus to introduce water vapour into the flowing gas streams.
In the prior art, the supplementary apparatus has been implemented in a number of ways. Bubbling the fuel or oxidant gases through heated water columns prior to introduction into the fuel cell has been applied. Alternatively, permeable membranes have been utilised as water transfer media such that water is carried into a gas stream from an adjacent plenum containing liquid water. Wicks have similarly been adopted to act as water transport media, liquid to vapour phase.
The additional apparatus may be separate from, or form an integral part of, the fuel cell stack. In either case, there is an associated increase in size and complexity of the assembly as a whole.
An alternative method is to deliver water directly to the membrane 11, 34, eg. directly to the electrode surfaces or into the channels 16 of the bipolar plates 31. This technique has the advantage of not only supplying the water to maintain a high membrane water content but also can act to cool the fuel cell through evaporation and extraction of latent heat of vaporisation.
This direct heat removal process that provides for the extraction of energy via the exit gas stream has distinct advantages associated with the elimination of intermediate cooling plates within the fuel cell stack assembly.
In the prior art, it is common to adopt a cooling regime which intersperses heat exchange plates between the electrochemically active plates so as to extract the thermal energy resulting from resistive and thermodynamic inefficiency of the fuel cell. These heat exchange, or cooling, plates utilise a recirculating or, less commonly, once-through fluid flow which carries heat away from the fuel cell stack. The cooling plates are in general of a different design to the active plates thereby adding to the complexity, size and cost of the fuel cell assembly.
A difficulty that can be encountered in the direct introduction of water is to deliver precise quantities of water to the many fluid flow fuel plate channels 16 within a fuel cell stack 30. Typically, this requires the delivery of precise quantities of water to many thousands of individual locations. To achieve this, a complex design of fluid flow field plate 14, 15 or 31 is required, which is more difficult to achieve and which increases costs of production.
If the water delivery process is uneven then the cooling effect can be poorly distributed, resulting in localised hot spots where overheating may result in physical stress and a deterioration of the membrane 11 mechanical properties and ultimately rupture. This effect applies with both poor (uneven) delivery across a plate surface and uneven delivery to each of the individual cells that make up the stack. In other words, temperature variations mail occur within a cell, or from cell to cell.